The present invention relates to a network of several single site trunked radio systems. Digital trunked radio transceivers capable of handling communications between numerous mobile units and dispatcher consoles in a single area are known. Trunked RF repeater systems are used, for example, by public service organizations (e.g., governmental entities such as counties, fire departments, police departments, etc.). Such RF repeater systems permit a relatively limited number of RF communications channels to be shared by a large number of users--while providing relative privacy to any particular RF communication (conversation). Typical state-of-the-art RF repeater systems are "digitally trunked" and use digital signals conveyed over the RF channels (in conjunction with digital control elements connected in the system) to accomplish "trunking" (time-sharing) of the limited number of RF channels among a large number of users.
Briefly, such digitally trunked RF communications systems include a "control" RF channel and multiple "working" RF channels. The working channels are used to carry actual communications traffic (e.g., analog FM, digitized voice, digital data, etc.). The RF control channel is used to carry digital control signals between the repeater sites and user RF transceivers (radio units) in the field. When a user's transceiver is not actively engaged in a conversation, it monitors the control channel for "outbound" digital control messages directed to it. User depression of a push-to-talk (PTT) switch results in a digital channel request message requesting a working channel (and specifying one or a group of callees) to be transmitted "inbound" over the RF control channel to the repeater site. The repeater site (and associated trunking system) receives and processes the channel request message.
Assuming a working channel is available, the repeater site generates and transmits a responsive "outbound" channel assignment digital message over the RF control channel. This message temporarily assigns the available working channel for use by the requesting transceiver and other callee transceivers specified by the channel request message. The channel assignment message automatically directs the requesting (calling) transceiver and callee transceivers to the available RF working channel for a communications exchange.
When the communication terminates, the transceivers "release" the temporarily assigned working channel and return to monitoring the RF control channel. The working channel is thus available for reassignment to the same or different user transceivers via further messages conveyed over the RF control channel. An exemplary "single site" trunked RF repeater system is disclosed in commonly-assigned U.S. Pat. Nos. 4,905,302 and 4,903,321 which are incorporated here by reference.
Single site trunked RF repeater systems may have an effective coverage area of tens of square miles. It is possible to provide one or more satellite receiving stations (and a single high power transmitting site) if a somewhat larger coverage area is desired. However, some governmental entities and other public service trunking system users may require an RF communications coverage area of hundreds of square miles. In order to provide such very large coverage areas, it is necessary to provide multiple RF repeater sites and to automatically coordinate all sites so that a radio transceiver located anywhere in the system coverage area may efficiently communicate in a trunked manner with other radio transceivers located anywhere in the system coverage area.
FIG. 1 is a schematic diagram of a simplified exemplary multiple-site trunked radio repeater system having three radio repeater (transmitting/receiving) sites S1, S2, and S3 providing rf communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 transmit signals to and receive signals from site S1; transceivers within area A2 transmit signals to and receive signals transmitted by site S2; and transceivers within area A3 transmit signals to and receive signals transmitted by site S3. Each repeater site S1, S2, S3 includes a set of repeating transceivers operating on a control channel and plural RF working channels. Each site typically has a site controller (e.g., a digital computer) that acts as a central point for communications in the site, and is capable of functioning relatively autonomously if all participants of a call are located within its associated coverage area.
To enable communications from one area to another a switching network, a "multisite switch" may be provided to establish audio and control signal pathways between repeaters of different sites. These pathways are set up at the beginning of each call and taken down at the end of each call. For example, the site controller (S1) receives a call from a mobile radio in A1 requesting a channel to communicate with a specific callee. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller S1 via an "inbound" digital control message transmitted over the RF control channel that a working or audio channel is requested. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the control channel to the audio channel assigned to the call. However, this assigned working channel is applicable only within the area covered by that site.
In addition, the site controller sends the channel assignment to multisite switch (200) which assigns an internal audio time slot to the call. The multisite switch also sends a channel request over a control messaging bus to other site controllers having a designated callee within their site area. Audio signals are routed through the multisite switch such that audio pathways are created to serve one or more callees and one or more dispatcher consoles 202 involved in the communication. Upon receiving a channel request, these "secondary" site controllers (in the sense they did not originate the call) assign an RF working channel to the call. Each secondary working channel is operative only in the area covered by the secondary site controller. The secondary site controller(s) also sends the channel assignment back up to the multisite switch.
Thus, the caller communicates with a radio unit or group of radio units in another area via the multisite switch. The call is initially transmitted to the primary site controller, routed through an assigned audio slot in the switch, and retransmitted by the secondary sites on various assigned channels in those other areas. When the call ends, the primary site controller deactivates the assigned channel for that site and notifies multisite switch 200 that the call is terminated. The multisite switch propagates an end of call command ("channel drop") to all other site controllers. This releases all working channels assigned to the call and breaks the associated audio routing pathways.
In addition to providing communications between mobile radio units in different areas, multisite switch 200 provides communications between land-line telephone subscribers through control telephone interconnect switch (CTIS) 212 and radio units as well as dispatchers and mobile radio units. Each dispatch console 202 may participate in calls in its area. Thus, when a call comes through the multisite switch from another area to a mobile radio, the switch informs the dispatch console 202 of the call in addition to notifying the corresponding site controller 102. The dispatch operator can then listen or participate in the call. Multisite switch 200 also handles calls to groups of mobile units and/or dispatch consoles by ensuring that the site controllers for all of the callees in the group assign a channel to the group call.
The multisite switch preferably includes a distributed control architecture. The logical functions and computational workload of the multisite switch are shared by various distributed microprocessor "nodes". Each node is connected either to a site controller 102, dispatch console 202, public and/or private landline telephone exchanges and other components of the particular communications system. Most nodes function as switch interfaces and include, for example, Master Interface Modules (MIMs) for nodes coupled to site controllers and Console Interface Modules (CIMs) for nodes coupled to dispatch consoles. Each interface module is supported by a controller card that utilizes several microprocessors. All of the cards have substantially the same hardware and are interchangeable. Each card acts as a gateway interface into the distributed control switch network. Detailed description and operation of such a distributed multi-site switch is set forth in commonly assigned U.S. Pat. No. 5,200,954 to Teel, Jr. et al. which is also incorporated herein by reference.
Trunked communication links between one or more multisite systems may be accomplished using dedicated Network Interface Modules (NIM) provided in the multisite switch to permit the interconnection and communication between multiple multisite switch controlled networks to create an extended overall communications network as described, for example, in commonly assigned U.S. patent application No. 08/156,785 entitled "Extended Trunked RF Communications System Networking" to Kent, the disclosure of which is incorporated herein by reference. In essence, the Network Interface Module allows a remote multisite switch controlled network to appear as just another node to the local multisite switch. Each network can then communicate over this common "node" interface permitting internetwork communication that is predominantly transparent to a network user. Moreover, using a plurality of network interface modules per switch, the overall communications network can be much extended by connecting individual multisite switch controlled networks together, for example, in series, "star" or multiple "star" configurations.
Permitting mobile radios to roam for one trunked rf communications site to another is known. For example, each site may allocate several "roaming ID's" to be temporarily assigned to roaming mobile radios that requests such roaming ID's. However, this procedure significantly reduces the already limited communication resources in each site by reserving some of those resources for roaming radios. This procedure is also inefficient because those reserved communication resources will go unused much of the time so they can be available to roaming radios. Moreover, a considerable amount of overhead is required to centrally track whether a radio is using its home selected ID or a temporarily assigned roaming ID.
Roaming between adjacent site areas may also be accomplished to the extent that a radio's personality (i.e., a preprogrammed memory in each radio) is preprogrammed with relevant site information including a current site identification number, control channel number, site priority, a site frequency set with control and working channel frequencies, specific talk groups, and a list that defines the "adjacencies" between site areas. "Adjacencies" refer to those sites that are physically located next to the currently selected site. For example, sites S2 and S3 are adjacencies for site S1 in FIG. 1. The adjacencies list permits the radio to know which sites are next to the site area in which it is presently located.
Unfortunately, such adjacency lists are static in the sense they are prestored in the radio's personality. However, to permit radios to roam freely over a wide area such as from one city to another city (requiring multiple multi-site switches) would require a large personality memory to store all potential site adjacency information for such a wide area. In fact, many users of trunked radio communications systems now desire coverage on the order of 100 or more sites, and even greater coverage is likely to be desired. Of course, there is are practical limits (e.g., size, cost, etc.) to the amount of adjacent site information that can be prestored in the radio personality. In addition, as the communications network extends, it becomes necessary to reuse channel numbers, frequency sets, site IDs, etc. as a radio roams out of range form one multi-site switch network to another.
Thus, there is a need for a system that permits a mobile radio to roam from site areas in one multi-site switch network to another multi-site switching network without having to know in advance an adjacency list for every conceivable adjacency configuration for every site in multiple multi-site networks. In such a system, radio units should only need a relatively small memory to store the most currently relevant site adjacency information. Moreover, reuse of trunked communication resources like site and channel numbers as well as frequency sets should be permitted.
In response to these needs and problems, the present invention provides for each radio a small, reprogrammable personality memory for storing the current relevant adjacency information given the roaming radio's present location, i.e., identification number, control channel number, and adjacent site index for an adjacent site. That adjacency "list" or "table" is continuously updated by adjacent site control channel messages received from the current site as the radio roams from site to site and from multisite switch to multisite switch.
Each site is provided with the adjacency information via messages over the extended multi-site switch networks. To accomplish this, distributed databases of adjacency control channel requirements for plural multi-site switch networks are created and maintained in each dedicated site interface node (MIM) and network interface node (NIM) of every multi-site switch included in the extended trunked communications network. The database maintains for each site defined in the extended area internal (the current or local multi-site switch) and external (other remote multi-site switches) adjacent site and control channel information. Each distributed node (a MIM) corresponding to a trunked communications site in the multisite switch network uses its database to report adjacency data to its trunked site. The database includes site, control channel and multisite switch number information for the current and adjacent sites. The network interface node (NIM) in each switch uses its database to report and request adjacency data to/from other multi-site switch networks.
A method in accordance with the present invention permits digitally trunked radio transceivers to roam over an extended coverage area including two or more digitally trunked radio frequency communication networks. Each network includes at least one digital repeater site having a corresponding site coverage area and serving digitally trunked radio transceivers disposed within or near the site coverage area. Each network also includes a local multisite switch for routing communications between the digital repeater sites within the two or more networks. Site adjacency information is established for each site and communicated between the networks.
A site adjacency list is maintained by each digitally trunked transceiver and modified as the radio transceiver roams into new coverage areas. More specifically, as the transceiver roams, it monitors the fidelity of communications received from a currently selected repeater site and an adjacent repeater site is included in the transceiver site adjacency list. The transceiver selects a new repeater site when the fidelity of communications received from a currently selected repeater site is less than that received from repeater sites included in the transceiver's site adjacency list.
A trunked communication system in accordance with the present invention permits portable/mobile radio units to roam from one geographical area to another over a wide area communications network. Plural trunked rf repeater site controllers located in associate geographical areas coordinate rf communications with radio units within or near their associated areas. Each radio unit dynamically selects a current site controller having the most optimal communications channel characteristics for conducting communications over the wide area network.
The network includes plural digital switches where each digital switch includes: a first bus for conveying digital audio information during preassigned time slots to interface modules connected to the first bus; a second bus for conveying operational control message information to interface modules connected to the second bus; site interface modules where each site interface module interfaces communications between the first bus and a corresponding one of the site controllers; and a network interface module for providing one digital switch with the capability that the transferred received both first and second bus information to/from another digital switch. Means are provided for transferring adjacency site information between the digital switches for updating adjacency site information as radio units roam over the wide area network.